A barrier film in which a thin layer of a metal oxide such as aluminum oxide, magnesium oxide, or silicon oxide is formed on the surface of a plastic substrate or a film has been widely used for packaging a product which requires blocking of various types of gases such as water vapor and oxygen or for packaging to prevent a quality change of, for example, food, industrial products or medical products.
Also, aside from the use for packaging, barrier films have been used as a substrate for liquid crystal displays, photoelectric conversion elements (also referred to as solar cells), or organic electroluminescent device substrates (also referred to as organic EL device substrates).
Aluminum foil has been widely used as a packaging material in this field, however, disposal after use is becoming a problem, and, in addition, since aluminum foil is basically an opaque material, it has the inherent problem that it is difficult to check the content from outside. Further, it is difficult to be used as a material for a solar cell which requires transparency.
Specifically, for a transparent substrate of which application to a liquid crystal display, an organic EL element or a solar cell is in progress, added has been a higher level of requirement, for example, capability of roll-to-roll production of the substrate, durability for a long duration, freedom of shape, and capability of curved display, in addition to the requirements of weight saving and a large scale. Replacement of a glass substrate, which is heavy, fragile and difficult in increasing the size with a film substrate such as a transparent plastic, is also in progress.
However, there has been a problem in that the gas barrier property of a film substrate such as a transparent plastic film is inferior to that of a glass substrate. For example, when such a substrate having an insufficient gas barrier property is used as a substrate of an organic photoelectric conversion element, water vapor or air may penetrate the substrate, which may cause degradation of the photoelectric conversion efficiency or durability due to deterioration of the organic film.
When a polymer substrate is used as a substrate of an electronic device, problems may occur that oxygen permeates the polymer substrate and soaks and spreads in an electron device to deteriorate the device, and that the degree of vacuum required in an electron device cannot be maintained.
In order to overcome such a problem, a technique to provide a thin metal oxide layer on a film substrate to obtain a gas barrier film substrate has been known. As a gas barrier film used for a packaging material or for a liquid crystal display, a plastic film on which a silicon oxide (for example, refer to Patent Document 1) or an aluminum oxide (for example, refer to Patent Document 2) is vacuum evaporated has been known.
As a method to form a film via a simple coating process, instead of a vacuum evaporation method which needs a vacuum process, there have been known several methods to form a gas barrier film composed of a converted silica film obtained by conducting a conversion treatment on a film formed by applying a coating liquid containing a silicon compound such as polysilazane (for example, refer to Patent Documents 3, 4 and 5).
Specifically, in Patent Document 4, disclosed has been a process in which a coated film of polysilazane is converted to a silica film via an oxygen plasma discharge treatment carried out under atmospheric pressure, in which formation of a gas barrier layer without using a vacuum system can be carried out.
However, the rate of water vapor permeation of the obtained film is 0.35 g/(m2·24 h), which cannot be said to be capable of using in a device such as described above. Generally, it is said that the rate of water vapor permeation desired for a gas barrier layer applied for an organic photoelectric conversion element is necessary to be much less than 1×10−2 g/(m2·24 h).
Also, as a method to form a gas barrier layer via an atmospheric pressure plasma discharge method, a film forming method in which high energy density and stable plasma discharge is possible has been proposed (for example, refer to Patent Documents 6 and 7).
In above mentioned Patent Documents 6 and 7, in order to prevent cracking which occurs when a silica film having a high barrier property is formed on a flexible plastic film, such as polyethylene terephthalate (PET), a stress relaxation design has been adopted, in which the barrier layer has a distribution of carbon content along the thickness direction of the layer and the hardness of the film becomes lower when coming closer to the substrate.
However, since it is a CVD film formation method, it has been revealed that, particles are generated in the plasma space as a byproduct, and that such particles adhere to the substrate, whereby uniform film formation may suffer. It is highly possible that particles generated in such a manner become a starting point of a barrier defect. Accordingly, this method cannot be fully recommended to be a method to stably form a uniform gas barrier layer.
When a gas barrier film having a certain extent of thickness is formed on a plastic film, the silica film is extremely brittle, when a high density silica film having a uniform density (or uniform hardness) along the thickness direction of the film is formed, whereby cracking frequently occurs. Accordingly, there has been a large limitation in the handling method or in a using method, which has been a big drawback. For example, a roll-to-roll process has not been able to be used as a method of forming a gas barrier layer, shipment conveyance in the roll shape has not been possible, and, when the gas barrier layer is used in an organic photoelectric conversion element, the element cannot be used on a curved surface. These have been the problems in the manufacturing of a gas barrier film employing a roll-to-roll method under atmospheric pressure.
However, it has been desired in this technical field to realize stable manufacturing of a uniform gas barrier film which is applicable to, for example, a substrate of an organic photoelectric conversion element, while satisfying both a high gas barrier property and cracking (or bending) resistance, employing a roll-to-roll method under atmospheric pressure.